The sense of feel is not typically used as a man-machine communication channel, however, it is as acute and in some instances as important as the senses of sight and sound, and can be intuitively interpreted. Tactile stimuli provide a silent and invisible, yet reliable and easily interpreted communication channel, using the human's sense of touch. Information can be transferred in various ways including force, pressure and frequency dependent mechanical stimulus. Broadly, this field is also known as haptics.
Haptic interfaces can be used to provide additional sensory feedback during interactive tasks. For example computer games make use of portable game consoles that can often include various motors and transducers that apply forces to the housing of the console at various vibrational rates and levels—these forces correlate to actions or activities within the game and improve the users gaming experience. Similar haptic interface techniques can be used for a variety of interface tasks including flat panel touch screens and mobile devices. Many human interface devices, for example a computer interface device, allow some form of haptic feedback to the user. Vibration feedback is more intuitive than audio feedback and has been shown to improve user performance.
A single vibrotactile transducer can be used for a simple application such as an alert. For example discrete temporal information may be provided in a message construct. A plurality of vibrotactile transducers can be used to provide more detailed information, such as direction information. For example, the spatial orientation of the person relative to some external reference can be provided by a body referenced vibrotactile cue. Using an intuitive body-referenced organization of vibrotactile stimuli, information can be communicated to a user. Such vibrotactile displays have been shown to reduce perceived workload by its ease in interpretation and intuitive nature (see for example: Rupert A H 2000, Tactile Situation Awareness System: Proprioceptive Prostheses for Sensory Deficiencies. Aviation, Space, and Environmental Medicine, Vol. 71(9):II, p. A92-A99). Tactile displays further can be used as a communication device for implementation under conditions of high physiological stress, such as the conditions associated with communication of army hand signals to personnel in vibrating vehicles or to dismounted soldiers under battle conditions (fatigue, stress and physical activity).
Tactile signals can be represented as variations in pulse, pulse length, amplitude, frequency and rhythm. There are well known limitations in the body's ability to resolve amplitude, frequency, pulse length and spatial acuity (especially for hairy skin).
The body's response to tactile stimuli is somewhat complex depending on stimulus characteristics, body location, transducer geometry and a large number of psychophysical factors. For example, the threshold of vibration detection vs. frequency (Bolanowski, S., Gescheider, G., Verrillo, R., and Checkosky, C. (1988). “Four channels mediate the mechanical aspects of touch”, J. Acoust. Soc. Am., 84(5), 1680-1694.
Bolanowski, S., Gescheider, G., and Verrillo, R. (1994). “Hairy skin: psychophysical channels and their physiological substrates”, Somatosensory and Motor Research, 11(3), 279-290.) is shown in FIG. 1 for two different skin classifications; smooth 12 (such as the hand) or hairy skin 10 (such as the torso). Sensitivity threshold for hairy skin 10 is about 20 dB higher in the high-frequency range than the glabrous skin 12 curve. There is also a “window” in the sensitivity threshold between about 200 and 400 Hz where the Pacinian (or “Pacinian like”) receptors in the body are most sensitive to vibration. This region is usually chosen for intended tactile signals.
Effective vibrotactile alert or communication display systems require both the detection and localization of the vibrotactile stimulus. Reasonable vibrotactor transducer requirements would include a vibration displacement amplitude that is equivalent to or above the human receptor system's sensitivity thresholds. It is known that the human sensitivity to vibration detection depends on body location and frequency. In practical vibrotactile display systems, it is also known that the vibrotactile detection threshold may vary with user physical activity, stress, fatigue and environmental conditions. Reasonable system technical requirements may include limiting the vibrotactile device frequency response to frequencies less than 300 Hz, vibrotactile devices that produce a displacement output that exceeds 24 dB above the threshold for sensitivity for a hairy skin body location (to account for noise), and vibrotactile devices that have a rise time of less than 5 ms (to avoid the accumulation of any delays or lags between the vibrotactile display and any the information provided by other display paths such as the audio or visual senses).
It is known that when a stimulus is sustained at some given level, a decrease with time is generally observed in the output of the activated nerve fibers. The decrease is usually a decrease in the rate of discharge (firing rate) of the neuron and it is accompanied by a decrease in the magnitude of the sensory response. This is known as adaptation. The sensory adaptation rate depends on the vibrotactile stimulus frequency and duration. Generally in tactile communication systems, it is important to keep vibrotactor durations considerably less than what would be required to cause adaptation. Alternatively, in systems where the vibrotactile stimulus is intended to be continuous, the displacement level of the stimulus must usually be kept below the sensory threshold to prevent adaptation.
Vibrotactile transducers can be wearable, mounted within a seat back and/or base, or included within an interface device such as a PDA or gaming interface. In each case, the requirements for the vibrotactile transducer are that the vibrational output be sufficient to illicit a strong, localized vibrotactile sensation (stimulus) to the body. These devices should preferably be small, lightweight, efficient, electrically and mechanically safe and reliable in harsh environments, and drive circuitry should be compatible with standard communication protocols to allow simple interfacing with various avionics and other systems.
Prior art describes a variety of alternative vibrotactile transducers (tactor) design configurations. The design challenge is to maximize vibrational output while minimizing size and weight in order for the device (or multiple devices) to be effectively applied against the body of a user. This trade-off is especially important in wearable vibrotactile applications. Often mechanical resonance is used to increase vibrotactile device efficiency with an associate narrowing or reduction in the usable frequency range.
It is known that sensitivity of skin receptors may be altered by the addition of a sub-sensory vibrational noise stimulus. Such mechanism is believed to be described by the phenomenon of stochastic resonance, whereby the cell sensory threshold for activation is reduced thereby enhancing the function of these detector cells and reducing the threshold of sensitivity. Prior art (U.S. Pat. No. 5,782,873) describes the use of stochastic resonance as a mechanism for stimulating sensory cells and enhancing receptor cell function. This patent teaches a method for introducing a sub-sensory band-limited noise signal to the sensory cell using vibration or electrical stimulation to the skin. Prior art further teaches that the stimulus should be approximately 90% of the sensory displacement sensitivity. Sub-sensory stimuli are presumed necessary as the body is known to adapt to continuous stimuli which would then render the stochastic resonance process ineffective.